An important factor enabling high reliability at electric power plants is to provide generators, such as high power generators in hydro or nuclear power plants, with condition monitoring systems in order to detect faults at an early stage. The invention aims to provide an improved diagnosing, detecting and protection method capable of detecting and identifying winding faults in a synchronous generator, especially inter-turn faults in a synchronous generator. Such internal short-circuit faults of a generator may lead to severe damage if not discovered and attended to. To detect inter-turn faults a few methods have been employed, including transverse differential protection and harmonic signature analysis.
The transverse differential protection is a widely-used method and provides good sensitivity and reliability for the synchronous generator inter-turn fault protection. This method utilizes the unbalanced or differential current between stator winding branches in each phase to indicate the existence of an inter-turn fault. For every winding that is monitored, the current through the winding in every branch of each phase is measured in both ends of the winding. Having dedicated measuring devices such as current transformers (CTs) in each end of each winding requires many measuring devices, thus increasing the overall cost of this type of protection. Sometimes, the synchronous generator stator winding branches are inside the machine due to manufacture preference and are not accessible for CT installation, and therefore limits the use of the method.
U.S. Pat. No. 7,528,611 (D1) describes a method and a system for detecting winding faults in a generator with parallel windings, especially inter-turn faults between the parallel windings. The system includes an arrangement of current transformers 24, 26, 28, 30, 32, and 34 on the terminal side of the generator, one current transformer for each coil branch (FIG. 1 in D1) with two branches in each phase. The current transformers are arranged to sense a difference between the coils of each phase, and output a split-phase current signal for each phase (abstract, column 5, line 60-column 6, line 2). The split-phase current signals are supplied to a microprocessor-based relay (38) adapted to detect inter-turn short circuit faults and output an alarm or a tripping signal (column 6, line 5-14). The system of D1 uses less current transformers than a conventional transverse differential protection, especially the system of D1 does not use current transformers on the neutral side of the generator. The system of D1 requires one current transformer for each coil winding of each phase. In practice it is often difficult to access the winding for installing a current transformer.
The harmonic signature analysis method utilizes harmonic spectrum of the synchronous generators voltages or currents to detect stator inter-turn fault. When stator inter-turn fault occurs, some characteristic harmonic components appear in the harmonic spectrum of the stator and rotor currents as the inter-turn fault signatures. However, the power electronic devices used in the power grid, such as converters or rectifiers, system disturbances and unbalanced operation of the external power grids may also induce harmonics that could influence the signatures and thus reduce the reliability of this method.
U.S. Pat. No. 7,592,772 describes inter-turn fault detection using an analysis of harmonics having a rotor search coil for measuring the rotor field winding current. The search coil is arranged at the field winding of the rotor. In industrial installations it is often difficult or impossible to install such a search coil. Also, when the frequency varies, the harmonic component cannot be accurately measured. The invention provides an alternative to such measurements and analysis of the rotor currents.
The article “A Robust On-Line Turn Fault Detection Technique for Induction Machines Based on Monitoring the Sequence Component Impedance Matrix” describes an inter-turn fault detection of induction motors based on a symmetrical sequence component impedance matrix. The changing of the off-diagonal elements of the symmetrical sequence impedances indicates the occurrence of inter-turn faults. In order to obtain data and parameters for the detection algorithm the method includes learning stages, which includes several intentional unbalanced operations. See the article: “S. Lee, R. Tallam, T. Habetler, IEEE Transaction on Power Electronics “A robust on-line turn fault detection technique for induction machines based on monitoring the sequence component impedance matrix,” vol. 18, no. 3, 2003”.
Such unbalanced learning stages are impractical for application on high power synchronous generators during their normal operation.